Nitrile rubber composition, vulcanizable nitrile rubber composition, and vulcanizate

ABSTRACT

A vulcanizable nitrile rubber composition comprising 100 parts by weight of a nitrile rubber, 0.1 to 20 parts by weight of a metal silicate and 0.1 to 10 parts by weight of a vulcanizing agent. Vulcanization of this vulcanizable rubber composition gives a vulcanizate exhibiting enhanced heating aging resistance when it is kept in a hot air atmosphere, especially at a high temperature.

TECHNICAL FIELD

This invention relates to a rubber vulcanizate exhibiting an enhancedheat aging resistance when it is kept in a hot air atmosphere; and anitrile rubber composition and a vulcanizable rubber composition, whichare used as a raw material for the production of the rubber vulcanizate.

BACKGROUND ART

A nitrile rubber represented by an acrylonitrile-butadiene copolymerrubber (NBR) has hitherto been widely used as an oil-resistant rubbermaterial in various fields. A highly saturated nitrile rubber preparedby reducing the amount of carbon-carbon unsaturated bond in the backbonechain was developed for satisfying a demand of placing on the market arubber vulcanizate having enhanced heat-aging resistance and hightensile strength.

Hydrogenated NBR has a structure such that butadiene units in themolecular chain have been hydrogenated and thus the content ofunsaturated bonds, i.e., double bonds, has been reduced to zero or anextremely small value. Therefore hydrogenated NBR has a remarkablyenhanced resistance to oxidative degradation and Is highly evaluated asa heat aging-resistant rubber.

However, the currently available highly saturated nitrile rubber stillhas unsaturated bonds even only in a minor amount, the heat agingresistance in a hot-air atmosphere is liable to be poor. Therefore,additives to be incorporated in the highly saturated nitrile rubber forenhancing the heat aging resistance are being developed.

For example, Japanese Unexamined Patent Publication No. H11-293039proposed the incorporation of a strong base, a salt of a strong basewith a weak acid, or a salt of a weak acid in highly saturated nitrilerubber to minimize the reduction of elongation at break as encounteredwhen the highly saturated nitrile rubber is kept in a hot-airatmosphere.

The incorporation of the proposed ingredients can reduce the reductionof elongation at break, but occasionally, the reduction of elongationcannot be minimized to a desired level. There is still a demand for theenhancement of heat aging resistance.

DISCLOSURE OF THE INVENTION

A primary object of the present invention is to provide a nitrile rubbervulcanizate exhibiting a minimized reduction of elongation at break evenwhen it is left to stand in a hot air atmosphere for a long period oftime.

The inventors conducted researches to achieve the above-mentionedobject, and found that incorporation of a metal silicate in a nitrilerubber gives a rubber vulcanizate exhibiting a minimized reduction ofelongation at break in a hot air atmosphere. Based on this finding, thepresent invention has been completed.

Thus, in a first aspect of the present invention, there is provided anitrile rubber composition comprising 100 parts by weight of a nitrilerubber and 0.1 to 20 parts by weight of a metal silicate.

In a second aspect of the invention, there is provided a vulcanizablerubber composition, which comprises the above-mentioned nitrile rubbercomposition, and further comprises 0.1 to 10 parts by weight, based on100 parts by weight of the nitrile rubber, of a vulcanizing agent.

Further, in a third aspect of the invention, there is provided avulcanizate obtained by vulcanizing the above-mentioned vulcanizablenitrite rubber composition.

BEST MODE FOR CARRYING OUT THE INVENTION

The nitrile rubber composition of the present invention comprises 100parts by weight of a nitrile rubber and 0.1 to 20 parts by weight of ametal silicate.

The nitrile rubber used in the present invention is a copolymer rubbermade by copolymerization of an α, β-ethylenically unsaturated nitrilemonomer with other monomer. The monomer to be copolymerized with an α,β-ethylenically unsaturated nitrile monomer is not particularly limited,and includes, for example, conjugated diene monomers, non-conjugateddiene monomers and α-olefin monomers. When an α, β-ethylenicallyunsaturated nitrile monomer is copolymerized with a conjugated dienemonomer, a copolymer rubber having a large iodine value tends to beproduced. However, according to the need, this copolymer rubber may behydrogenated whereby the carbon-carbon double bond is saturated and theiodine value is lowered.

The α, β-ethylenically unsaturated nitrile monomer is not particularlylimited, and, as specific examples thereof, there can be mentionedacrylonitrile, methacrylonitrile and α-chloroacrylonitrile. Of these,acrylonitrile is preferable. The content of units of the α,β-ethylenically unsaturated nitrile monomer in the nitrile rubber ispreferably in the range of 10 to 60% by weight, more preferably 12 to55% by weight and especially preferably 15 to 50% by weight.

The conjugated diene monomer is also not particularly limited, and, asspecific examples of the conjugated diene monomer, there can bementioned 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and1,3-pentadiene. Of these, 1,3-butadiene is preferable.

The non-conjugated diene monomer is also not particularly limited andpreferably includes those which have 5 to 12 carbon atoms. As specificexamples of the non -conjugated diene monomer, there can be mentioned1,4-pentadiene, 1,4-hexadiene, vinylnorbornene and cyclopentadiene.

The α-olefin monomer is also not particularly limited and preferablyincludes those which have 2 to 12 carbon atoms, and, as specificexamples thereof, there can be mentioned ethylene, propylene, 1-butene,4-methyl-1-pentene, 1-hexene and 1-octene.

Other monomers can be copolymerized with an α, β-ethylenicallyunsaturated nitrile monomer, provided that a substantially adverseinfluence is not exerted upon the effect of the present invention. Asexamples of such monomers, there can be mentioned α, β-ethylenicallyunsaturated monocarboxylic acids, α, β-ethylenically unsaturateddicarboxylic anhydrides, α, β-ethylenically unsaturated carboxylic acidesters, aromatic vinyl monomers, and fluorine-containing vinyl monomers.Copolymerizable antioxidants may also be used. .

The α, β-ethylenically unsaturated monocarboxylic acids include, forexample, acrylic acid and methacrylic acid. The α, β-ethylenicallyunsaturated dicarboxylic anhydrides include, for example, itaconicanhydride, fumaric anhydride and maleic anhydride.

As specific examples of the α, β-ethylenically unsaturated carboxylicacid esters, there can be mentioned methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, n-dodecyl acrylate,n-dodecyl methacrylate, methoxymethyl acrylate, methoxymethylmethacrylate, α-cyanoethyl acrylate, α-cyanoethyl methacrylate,β-cyanoethyl acrylate, β-cyanoethyl methacrylate, cyanobutyl acrylate,cyanobutyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, monoethyl maleate, mono-n-butyl itaconate, dimethylmaleate, dimethyl fumarate, diethyl itaconate, dimethylaminomethylacrylate, dimethylaminomethyl methacrylate, diethylaminomethyl acrylate,diethylaminomethyl methacrylate, trifluoroethyl acrylate, trifluoroethylmethacrylate, tetrafluoroethyl acrylate, tetrafluoroethyl methacrylate,fluorobenzyl acrylate and fluorobenzyl methacrylate.

As specific examples of the aromatic vinyl, there can be mentionedstyrene, α-methylstyrene, vinylpyridine and o-trifluoromethylstyrene. Asspecific examples of the fluorine-containing vinyl, there can bementioned fluoroethyl vinyl ether, fluoropropyl vinyl ether,difluoroethylene and tetrafluoroethylene. As specific examples of thecopolymerizable antioxidant, there can be mentioned N-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide,N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)-crotonamide,N-phenyl-4-(3-vinylbenzyloxy)aniline andN-phenyl-4-(4-vinylbenzyloxy)aniline.

The Mooney viscosity (ML₁₊₄, 100°C.) of the nitrile rubber used in thepresent invention is not particularly limited, but is preferably in therange of 20 to 140 and more preferably 30 to 120. If the Mooneyviscosity is too large, the mechanical strengths are low. In contrast,if the Mooney viscosity is too large, the processability becomes poor.

The iodine value of the nitrile rubber used in the present invention isalso not particularly limited, but is preferably not larger than 120,more preferably not larger than 60 and especially preferably not largerthan 30. Iodine value is a measure indicating the amount of unsaturatedbonds in the backbone of polymer. If the iodine value is too large, thehot-air aging resistance is low. In the case when the iodine value mustbe lowered, the polymer is hydrogenation whereby unsaturated bonds inthe molecule chain of rubber are saturated as mentioned above.

The method of producing the nitrile rubber used in the present inventionis not particularly limited. The nitrile rubber can be produced by theconventional polymerization procedure, followed by, if desired,hydrogenation by the conventional hydrogenation procedure.

The polymerization procedure is also not particularly limited. Thenitrile rubber is produced usually by radical copolymerization of theabove-mentioned monomers. The polymerization may be carried out by aconventional procedure, which may be any of emulsion polymerization,suspension polymerization and solution polymerization procedures. Ofthese, an emulsion polymerization procedure is preferable.

Any limitation is not imposed to the optional hydrogenation of nitrilerubber, and the conventional hydrogenation procedure can be employed forselectively hydrogenating unsaturated bonds in the backbone of polymer.

The procedure of recovering nitrite rubber after the completion ofpolymerization or hydrogenation is also not particularly limited, andany known procedure can be employed provided that a substantialalteration of the properties of nitrite rubber can be avoided.

The metal silicate used in the present invention is a compound selectedfrom those which are represented by the following general formula 1 to3. Of these, a compound of general formula 1 is preferable.M¹ ₂O.xSiO₂.nH₂O  General formula 1:wherein x is a positive number, n is zero or a positive integer, and M¹is a metal element having an oxidation number of 1,M²O.ySiO₂.mH₂O  General formula 2:wherein y is a positive number, m is zero or a positive integer, and M²is a metal element having an oxidation number of 2.M³ ₂O₃.zSiO₂.pH₂O  General formula 3:wherein z is a positive number, p is zero or a positive integer, and M³is a metal element having an oxidation number of 3.

In general formula 1, M¹, that is a metal element having an oxidationnumber of 1, includes, for example, sodium and potassium. Of these,sodium is preferable. In general formula 2, M², that is a metal elementhaving an oxidation number of 2, includes, for example, magnesium,calcium and iron. In general formula 3, M³, that is a metal elementhaving an oxidation number of 3, includes, for example, boron andaluminum.

As specific examples of the metal silicate, there can be mentioned, asthose of general formula 1, metal orthosilicates such as sodiumorthosilicate, sodium orthosilicate hydrate and potassium orthosilicate,and metal metasilicates such as sodium metasilicate pentahydrate andsodium metasilicate nonahydrate; as those of general formula 2,magnesium silicate, magnesium silicate hydrate, calcium silicate andcalcium silicate hydrate; and, as those of general formula 3, boronsilicate, boron silicate hydrate, aluminum silicate and aluminumsilicate hydrate.

The amount of metal silicate in the nitrile rubber composition of thepresent invention is in the range of 0.1 to 20 parts by weight,preferably 0.3 to 15 parts by weight and especially preferably 0.5 to 10parts by weight, based on the weight of the nitrile rubber. If theamount of metal silicate is too small, the rubber vulcanizate has poorheat aging resistance. In contrast, if the amount of metal silicate istoo large, the rubber composition has too small processability onkneading, and therefore, becomes difficult to shape.

Ingredients which are conventionally used in the rubber industry can beincorporated in the nitrile rubber composition of the present invention.The ingredients include, for example, a reinforcing filler such ascarbon black and silica, a non-reinforcing filler such as calciumcarbonate and clay, a processing aid, a plasticizer, an antioxidant, anantiozonant and a colorant. The amount of these ingredients is notparticularly limited provided that a substantial influence is notexerted on the object and effect of the present invention, and theamount thereof can appropriately be chosen depending upon. theparticular object thereof.

Any limitation is not imposed on the method by which a metal silicateand optional other ingredients are incorporated in the nitrile rubbercomposition. A method of mixing and kneading together a nitrile rubberand a metal silicate by a roll or an internal mixer to give a nitrilerubber composition is preferable. Further, there can be employed amethod of adding a metal silicate in a polymerization liquid duringpolymerization for the production of a nitrile rubber, and, after thetermination of polymerization, the metal silicate is recovered togetherwith the nitrile rubber to yield a nitrile rubber composition; and amethod of adding a metal silicate in a polymerization liquid when orafter the polymerization is terminated, and, after the polymerizationliquid is thoroughly stirred, the metal silicate is recovered togetherwith the nitrite rubber to yield a nitrile rubber composition.

In the case where hydrogenation of a nitrile rubber is carried outaccording to the need, a metal silicate can be incorporated in areaction liquid during or after completion of the hydrogenation, and themetal silicate is recovered together with the nitrile rubber to yield anitrile rubber composition. It is to be noted, however, that a metalsilicate cannot be occasionally added before or during the hydrogenationbecause the hydrogenation reaction is impeded depending upon theparticular combination of the metal silicate and a hydrogenationcatalyst. Further, in some cases, it is difficult or even impossible torecover a metal silicate together with a nitrile rubber, or it ispossible that a metal silicate is recovered only with a low yield.Therefore, an appropriate method of incorporating a metal silicateshould be chosen depending upon the particular procedures ofpolymerization, hydrogenation and recovery.

The vulcanizable rubber composition of the present invention comprises100 parts by weight of a nitrile rubber, 0.1 to 20 parts by weight of ametal silicate and 0.1 to 10 parts by weight of a vulcanizing agent.That is, the vulcanizable nitrile rubber composition comprises theabove-mentioned nitrile rubber composition and a vulcanizing agent. Thevulcanizable rubber composition is made into a vulcanizate by heating.

The vulcanizing agent used is not particularly limited, and it includesknown sulfur and sulfur-containing vulcanizing agents, and organicperoxide vulcanizing agents. As specific examples of the sulfur andsulfur-containing vulcanizing agent, there can be mentioned sulfur suchas powdery sulfur and precipitated sulfur; and organic sulfur compoundssuch as 4,4′-dithiomorpholine, tetramethylthiuram disulfide,tetraethylthiuram disulfide, and a high-molecular-weight polysulfide.

As the organic peroxide vulcanizing agent, those which are used as avulcanizing agent in a rubber industry can be used. The organic peroxidevulcanizing agent includes, for example, dialkyl peroxides, diacylperoxides and peroxy esters. Of these, dialkyl peroxides are preferable.As specific examples of the organic peroxide vulcanizing agent, therecan be mentioned dialkyl peroxides such as dicumyl peroxide, di-t-butylperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and1,3-bis(t-butylperoxyisopropyl)benzene; diacyl peroxides such as benzoylperoxide and isobutyryl peroxide; and peroxy esters such as2,5-dimethyl-2,5-bis(benzoylperoxy)hexane and t-butylperoxyisopropylcarbonate.

The amount of vulcanizing agent is in the range of 0.1 to 10 parts byweight, preferably 0.2 to 7 parts by weight and especially preferably0.3 to 5 parts by weight, based on 100 parts by weight of nitrilerubber.

More specifically, the amount of sulfur or sulfur-containing vulcanizingagent is preferably in the range of 0.2 to 7 parts by weight andespecially preferably 0.3 to 5 parts by weight, based on 100 parts byweight of nitrile rubber. The sulfur and sulfur-containing vulcanizingagent are preferably used in combination with a vulcanization aid. Apreferable vulcanization aid includes known vulcanization aids such aszinc oxide, a guanidine vulcanization accelerator, a thiazolevulcanization accelerator, a thiuram vulcanization accelerator and adithiocarbamate vulcanization accelerator. As specific examples of thevulcanization accelerator, there can be mentioned guanidinevulcanization accelerators such as diphenylguanidine,diorthotolylguanidine and orthotolylbiguanidine; thiazole vulcanizationaccelerators such as 2-mercaptobenzothiazole and dibenzothiazyldisulfide; thiuram vulcanization accelerators such as tetramethylthiuramdisulfide, tetraethylthiuram disulfide andN,N′-dimethyl-N,N′-diphenylthiuram disulfide; and dithiocarbamatevulcanization accelerators such as tellurium dimethyldithiocarbamate andzinc dimethyldithiocarbamate.

The amount of the organic peroxide vulcanizing agent is preferably inthe range of 0.3 to 7 parts by weight and especially preferably 0.5 to 5parts by weight, based on 100 parts by weight of nitrile rubber. Theorganic peroxide vulcanizing agent is usually used in combination with avulcanization aid. A preferable vulcanization aid includes knownvulcanization aids such as zinc oxide, magnesium oxide, triallylcyanurate, trimethylolpropane trimethacrylate and N,N′-m-phenylenebismaleimide. The vulcanization aid may be used as a dispersion preparedby dispersing it in a filler such as clay, calcium carbonate or silicawhereby a vulcanizable nitrile rubber composition having an enhancedprocessability can be obtained.

The kind and amount of vulcanization aid are not particularly limitedand can be appropriately chosen depending upon the particular use of andproperties required for a vulcanizate, and the particular kind ofvulcanizing agent.

The method by which the vulcanizable nitrile rubber composition isprepared is not particularly limited. However, vulcanizing agents andvulcanization aids which are activated by heat must be incorporated witha nitrile rubber under conditions giving no heat to the vulcanizingagents and vulcanization aids. Usually a vulcanizing agent and avulcanization aid are incorporated in and kneaded together with anitrile rubber composition by a roll or other mixing and kneading meansat a temperature lower than the vulcanization initiating temperature.When a vulcanization aid incapable of being activated by heat is usedfor the preparation of a vulcanizable nitrile rubber composition, thevulcanization aid can be mixed and kneaded together with otheringredients under heated conditions.

The vulcanizate of the present invention is obtained by vulcanizing theabove-mentioned vulcanizable rubber composition. The procedure by whichthe vulcanizable rubber composition is vulcanized is not particularlylimited. The vulcanization can be carried out by heating either after orsimultaneously with shaping of the vulcanizable rubber composition.

The vulcanization temperature is preferably in the range of 100 to 200°C., more preferably 130 to 190° C. and especially preferably 140 to 180°C. When the vulcanization temperature is too low, a substantially longtime is required for vulcanization or the vulcanization density tends tobe low. In contrast, when the vulcanization temperature is too high,defective moldings are liable to be produced.

The vulcanization time varies depending upon the particular vulcanizingprocedure, vulcanization temperature and shape of a vulcanizate, but ispreferably chosen in a range of one minute to 5 hours.

The vulcanization procedure can be appropriately chosen from those whichare conventionally employed in vulcanization of rubbers, and include,for example, press heating, vapor heating, oven heating and hot airheating.

The present invention will now be described specifically by thefollowing examples and comparative examples. The physical propertieswere measured as follows.

Iodine value was measured according to JIS K6235.

Mooney viscosity was measured at 100° C. according to JIS K6300.

Tensile strength, elongation and 100% tensile modulus of a vulcanizatewere measured according to JIS K6251. Hardness (Duro A) of a vulcanizatewas measured according to JIS K 6253.

Accelerated aging test in hot air was carried out by allowing a specimento stand at 150° C. in air for 168 hours, 336 hours or 504 hours, or at175° C. in air for 168 hours, 336 hours or 504 hours. Elongation wasmeasured before and after the accelerated aging, and elongation changewas calculated according to the formula:Elongation change (%)=[(B−A)/A]×100wherein A and B are elongations as measured before and after theaccelerated aging, respectively.

EXAMPLE 1

To 100 parts by weight of a hydrogenated acrylonitrile-butadienecopolymer rubber (acrylonitrile unit content: 36.2% by weight, iodinevalue: 4, Mooney viscosity: 65, tradename “Zetpol™ 2000L” available fromZeon Corporation, hereinafter referred to “nitrile rubber A”), wereadded 1 part by weight of octylated diphenylamine (antioxidant), 0.4part by weight of a zinc salt of mercaptomethylbenzimidazole(antioxidant), 2 parts by weight of magnesium oxide (vulcanization aid),2 parts by weight of zinc oxide (vulcanization aid), 35 parts by weightof wet white carbon (tradename “Carplex™ #1120” available from Shionogi& Co., Ltd.), 5 parts by weight of tri-(2-ethylhexyl) trimellitate(plasticizer), 1.5 parts by weight of triallyl isocyanurate(vulcanization aid), 1 part by weight ofvinyltris(β-methoxyethoxy)silane and tetrasodium 1-silicate hydrate (3parts by weight In Example 1, 6 parts by weight in Example 2 and 9 partsby weight in Example 3). The combined ingredients were mixed and kneadedtogether by a small-size Banbury mixer. To the kneaded mixture, 8 partsby weight of 2,2′-bis(terbutylperoxydiisopropyl)benzene (40% by weightgrade; organic peroxide vulcanizing agent, tradename “Vul-cup™ 40KE”available from Hercules Co.) was added, and the mixture was kneadedtogether by a roll to give a vulcanizable nitrile rubber composition.

The vulcanizable nitrile rubber composition was press-cured at 170° C.for 20 minutes to give a sheet-form vulcanizate having a thickness of 2mm. Dry mechanical strengths and elongation changes after acceleratedaging in hot air of the vulcanizate were evaluated. The results areshown in Table 1.

REFERENCE EXAMPLE 1

A reactor was charged with 2 parts by weight of potassium oleate(emulsifier), 0.1 part by weight of potassium phosphate (stabilizer) and150 parts by weight of water. Then 20 parts by weight of acrylonitrile,15 parts by weight of butyl acrylate, 35 parts by weight of1,3-butadiene and 0.45 parts by weight of t-dodecyl mercaptan (molecularweight modifier) were added, and further, 0.015 part by weight offerrous sulfate (activator) and p-menthane hydroperoxide (polymerizationinitiator) to initiate emulsion polymerization at 10° C. When thepolymerization conversion reached 60%, 10 parts by weight ofacrylonitrile, 10 parts by weight of butyl acrylate and 10 parts byweight of 1,3-butadiene were added. Further, when the polymerizationconversion reached 85%, 0.2 part by weight, per 100 parts by weight ofthe total monomers, of hydroxylamine sulfate was added to terminate apolymerization reaction. During polymerization, a slight bit of thepolymerization liquid was taken at every 3% increase of thepolymerization conversion to determine the content of each monomer inthe small part of polymer. Subsequent to the termination ofpolymerization, the polymerization liquid was heated and subjected tosteam distillation at 70° C. under a reduced pressure to recoverunreacted monomers. Then 2 parts by weight of an alkylated phenol as anantioxidant was added to the polymerization liquid to give a copolymerlatex.

The obtained copolymer latex was dropwise added to 3,000 parts by weightof an aqueous coagulating liquid maintained at 50° C. and havingdissolved therein 3 parts by weight of calcium chloride as a coagulatingagent whereby the copolymer latex was coagulated into a crumb. The crumbwas washed with water and then dried at 50° C. under a reduced pressure.

The thus-obtained acrylonitrile-butyl acrylate-butadiene polymer wasdissolved in methyl isobutyl ketone, and hydrogenated using apalladium/silica catalyst in a pressure vessel.

The hydrogenated acrylonitrile-butyl acrylate-butadiene terpolymer(hereinafter referred to “nitrile rubber B”) had an acrylonitrile unitcontent of 30% by weight, a butyl acrylate unit content of 25% byweight, an iodine value of 15, and a Mooney viscosity of 80. Theextrapolated glass transition initiating temperature and extrapolatedglass transition terminating temperature as measured by a differentialscanning calorimeter were −39.1° C. and 31.3° C., respectively, namely,the temperature difference therein was below 10° C.

The above-mentioned monomer unit contents in nitrile rubber B weredetermined according to ¹H-NMR and a nitrogen content determiningprocedure according to a semi-micro-kjeldahl method. It was confirmedthat each of the thus-measured monomer unit contents was in agreementwith the difference between the amount of monomer used forpolymerization and the amount of monomer remained unreacted.

EXAMPLE 4

Following the same procedures as employed in Example 1, a vulcanizatewas made except that nitrile rubber B prepared in Reference Example 1was used instead of nitrile rubber A. Dry mechanical strengths andelongation changes after accelerated aging in hot air of the vulcanizatewere evaluated. The results are shown in Table 1.

EXAMPLE 5

Following the same procedures as employed in Example 1, a vulcanizatewas made except that a hydrogenated acrylonitrile-butylacrylate-butadiene terpolymer (acrylonitrile unit content: 44% byweight, iodine value: 15, Mooney Viscosity: 75; hereinafter referred to“nitrile rubber C”) was used instead of nitrile rubber A. Dry mechanicalstrengths and elongation changes after accelerated aging in hot air ofthe vulcanizate were evaluated. The results are shown in Table 1.

TABLE 1 Examples 1 2 3 4 5 Formulation of rubber composition (parts byweight) Nitrile rubber A 100 100 100 — — Nitrile rubber B — — — 100 —Nitrile rubber C — — — — 100 Tetrasodium 1-silicate 3 6 9 3 3 hydrateTensile test Tensile strength (MPa) 19.2 18.3 17.8 18.2 18.9 Elongation(%) 540 550 540 330 480 100% tensile stress 2.20 1.90 1.70 1.50 1.60(MPa) Hardness (Duro A) 71 70 70 67 70 Elongation change in acceleratedaging in hot air (%) 150° C. 168 hours 7 6 9 5 8 336 hours 8 12 14 13 10504 hours 3 6 10 4 5 175° C. 168 hours −8 −8 −9 −4 −4 336 hours −2 13 16−1 0 504 hours −43 −38 −35 −36 −38

COMPARATIVE EXAMPLE 1

Following the same procedures as employed in Examples 1 to 3 vulcanizatewas made except that sodium silicate was not incorporated in a nitrilerubber composition. Dry mechanical strengths and elongation changesafter accelerated aging in hot air of the vulcanizate were evaluated.The results are shown in Table 2.

COMPARATIVE EXAMPLES 2 and 3

Following the same procedures as employed in Examples 1 to 3,vulcanizates were made except that 1 part by weight or 3 parts by weightof sodium carbonate was used instead of sodium silicate. Dry mechanicalstrengths and elongation changes after accelerated aging in hot air ofthe vulcanizate were evaluated. The results are shown in Table 2.

COMPARATIVE EXAMPLE 4

Following the same procedures as employed in Example 4, a vulcanizatewas made except that sodium silicate was not incorporated in a nitrilerubber composition. Dry mechanical strengths and elongation changesafter accelerated aging in hot air of the vulcanizate were evaluated.The results are shown in Table 2.

COMPARATIVE EXAMPLE 5

Following the same procedures as employed in Example 5 a vulcanizate wasmade except that sodium silicate was not incorporated in a nitrilerubber composition. Dry mechanical strengths and elongation changesafter accelerated aging in hot air of the vulcanizate were evaluated.The results are shown in Table 2.

TABLE 2 Comparative Examples 1 2 3 4 5 Formulation of rubber composition(parts by weight) Nitrile rubber A 100 100 100 — — Nitrile rubber B — —— 100 — Nitrile rubber C — — — — 100 Sodium carbonate — 1 3 — — Tensiletest Tensile strength (MPa) 23.9 21.2 20.8 20.4 22.4 Elongation (%) 530510 500 360 490 100% tensile stress 2.70 2.80 2.90 2.10 2.20 (MPa)Hardness (Duro A) 72 71 71 68 71 Elongation change in accelerated agingin hot air (%) 150° C. 168 hours −6 9 12 −5 −8 336 hours −18 9 15 −15−22 504 hours −24 6 18 −20 −32 175° C. 168 hours −8 0 −6 −4 −10 336hours −43 −6 12 −39 −53 504 hours −95 −96 −93 −93 −100

The vulcanizate made in Comparative Example 1 wherein a metal silicatewas not incorporated in a rubber composition exhibited a largeelongation change under a heat load especially at a high temperature ,i.e., 175° C. than those of vulcanizates made in Examples 1-3 whereinthe same nitrile rubber A was used and a metal silicate was incorporatedin respective rubber compositions.

The vulcanizates made in Comparative Examples 2 and 3 wherein sodiumcarbonate was incorporated instead of a metal silicate in rubbercomposition exhibited a large elongation change under a heat loadespecially at a high temperature, i.e., 175° C., and for many hours,i.e., 504 hours, than those of vulcanizates made in Examples 1-3 whereinthe same nitrile rubber A was used and a metal silicate was incorporatedin respective rubber compositions.

The vulcanizates made in Comparative Examples 4 and 5 wherein a metalsilicate was not incorporated in a rubber composition exhibited a largeelongation change under a heat load than those of vulcanizates made inExamples 4 and 5 wherein the same nitrile rubbers B and C were used,respectively, and a metal silicate was incorporated in respective rubbercompositions.

Industrial Applicability

The vulcanizate of the present invention has excellent resistance toheat aging under hot air atmosphere. Making efficient use of thisproperty, the vulcanizate is useful as articles used in hot airatmosphere for many hours, for example, automobile parts such as anO-ring, a seal, a packing, a gasket, a diaphragm, a hose and a belt.

1. A nitrile rubber composition comprising 100 parts by weight of anitrile rubber having an iodine value of not larger than 60 and 0.3 to15 parts by weight of a metal silicate represented by the followinggeneral formula 1M ¹ ₂O.xSiO₂.nH₂O  general formula 1: wherein x is a positive number, nis zero or a positive integer, and M¹ is a metal element having anoxidation number of 1; wherein the total of all rubber ingredients inthe nitrile rubber composition is 100 parts by weight.
 2. The nitrilerubber composition according to claim 1, wherein the nitrile rubbercontains 10% to 60% by weight of α,β-ethylenically unsaturated nitrilemonomer units.
 3. The nitrile rubber composition according to claim 1,wherein the nitrile rubber has been hydrogenated.
 4. The nitrite rubbercomposition according to claim 1, wherein the metal silicate is a metalorthosilicate or a metal metasilicate.
 5. The nitrite rubber compositionaccording to claim 1, wherein the metal silicate is selected from sodiumorthosilicate, sodium orthosilicate hydrate, potassium orthosilicate,sodium metasilicate pentahydrate, sodium metasilicate nonahydrate,magnesium silicate, magnesium silicate hydrate, calcium silicate,calcium silicate hydrate, boron silicate, boron silicate hydrate,aluminum silicate and aluminum silicate hydrate.
 6. The nitrile rubbercomposition according to claim 1, wherein the metal silicate is presentin an amount of 0.5 to 10 parts by weight based on 100 parts by weightof the nitrile rubber.
 7. A vulcanizable rubber composition, whichcomprises the rubber composition as claimed in claim 1, and furthercomprises 0.1 to 10 parts by weight, based on 100 parts by weight of thenitrile rubber, of a vulcanizing agent.
 8. The vulcanizable nitrilerubber composition according to claim 7, wherein the vulcanizing agentis sulfur or a sulfur-containing vulcanizing agent, or an organicperoxide vulcanizing agent.
 9. The vulcanizable nitrile rubbercomposition according to claim 7, wherein the vulcanizing agent issulfur or a sulfur-containing vulcanizing agent, and its amount is 0.2to 7 parts by weight, based on 100 parts by weight of the nitrilerubber.
 10. The vulcanizable nitrile rubber composition according toclaim 7, wherein the vulcanizing agent is an organic peroxidevulcanizing agent and its amount is 0.3 to 7 parts by weight, based on100 parts by weight of the nitrile rubber.
 11. The vulcanizable nitrilerubber composition according to claim 8, which further comprises avulcanizing aid.
 12. The nitrile rubber composition according to claim1, wherein the nitrile rubber has an iodine value of not larger than 30.13. A vulcanizate obtained by vulcanizing the vulcanizable nitrilerubber composition as claimed in claim
 7. 14. The vulcanizate accordingto claim 13, which is an automobile part selected from an O-ring, aseal, a packing, a gasket, a diaphragm, a hose or a belt.